1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device comprising an array of memory cells each including an antifuse to store information based on a variation in resistance in accordance with destruction of the insulator in the antifuse.
2. Description of the Related Art
A semiconductor integrated circuit essentially requires a nonvolatile OTP (One Time Programmable) memory capable of continuously retaining stored information even after power-off. The uses thereof widely spread over the redundancy use in mass storage memories such as DRAMs and SRAMs, the use for tuning analog circuits, the use for storage of codes such as encryption keys, and the use for storing manufacture histories.
Previously, the memory redundancy has employed a ROM that includes a laser fuse. Such the laser fuse ROM, however, requires a special fuse blower and a blowing step using the blower and accordingly has a drawback associated with a high programming cost. The minimum dimension of the laser fuse is determined from the wavelength of the laser light used. Therefore, fine patterning can not be synchronized with other circuit parts, and the proportion of the occupied area gradually increases as a problem. Further, as programming is executed using a laser, programming can be executed only in a wafer state. Accordingly, it can not be used to relieve a failure found in a product test after packaging as a drawback. Therefore, there have been higher expectations to nonvolatile semiconductor memory devices electrically programmable without the use of any laser blower.
The electrically programmable nonvolatile semiconductor memory devices include a MOS-structured antifuse (see, for example, JP 5-226599A) . In writing, a high voltage is applied across the antifuse to destruct an insulator to store one-bit information. In reading, a low voltage incapable of causing destruction of the insulator is applied across the antifuse to sense the presence/absence of destruction of the antifuse from the value of current flowing in the antifuse, thereby reading out one-bit information. In this way, the antifuse, because writing and reading can be simply executed with the application of a voltage across the antifuse, is one of nonvolatile semiconductor memory devices most expectedly available in the future.
In general, the insulator before destruction has an extremely large resistance of around 100 MΩ. The destructed insulator also has a large resistance of around 100 KΩ to 1 MΩ, however, and accordingly the application of a voltage of, for example, around 1 V on reading merely causes a small current of 1-10 μA flowing in the antifuse. This makes it difficult to sense the presence/absence of destruction of the antifuse in a short time, that is, to read information at a high speed. A solution needs an antifuse that allows a sufficiently large read current to flow therein.